While you do not need a fireplace in a Passive House to keep warm, sometimes even building physicists like to be romantic and gaze at a flame. So, if you don’t mind the extra money, why not install a fireplace in your Passive House? Of course you need a clean, highly efficient system, but that is not enough.
To operate a fireplace safely in a Passive House, 4 conditions must be met:
1. The fireplace must be ventilated independently (external air supply)
2. The whole system (fireplace/flues) must be airtight
3. A flap valve in the flue must prevent back drafts
4. A differential pressure switch must switch off the ventilation system, if large pressure differentials exist (in case of a malfunction).
Without these safeguards in place, there is a risk of suffocation when fires are operated in conjunction with a ventilation system.

Check here for an example that meets condition 1 and 2 (sorry, German only):